Microelectronic elements often comprise a thin slab of a semiconductor material, such as silicon or gallium arsenide, commonly called a semiconductor chip or die. Dies are commonly provided as individual, prepackaged units. In some unit designs, the die is mounted to a substrate or a chip carrier, forming a microelectronic structure or package, for example, which may in turn be mounted on a circuit panel, such as a printed circuit board (PCB).
As integrated circuit (IC) chip technology matures, smaller packages are possible due to smaller and denser ICs. Many packages include solder masses in the form of solder balls that are typically between about 0.1 mm and about 0.8 mm (5 and 30 mils) in diameter, and are attached to the terminals of the package. A package having an array of solder balls projecting from its bottom surface (e.g., surface opposite the front face of the die) is commonly referred to as a ball grid array or “BGA” package. Other packages, referred to as land grid array or “LGA” packages are secured to the substrate by thin layers or lands formed from solder. Packages of this type can be quite compact. Certain packages, commonly referred to as “chip scale packages,” occupy an area of the circuit board equal to, or only slightly larger than, the area of the device incorporated in the package. This scale is advantageous in that it reduces the overall size of the assembly and permits the use of short interconnections between various devices on the substrate, which in turn limits signal propagation time between devices and thus facilitates operation of the assembly at high speeds.
However, as carriers and packages (i.e., fan out wafer level packaging (FOWLP), coreless packages, etc.) become smaller, they are also becoming thinner, as well as the associated substrates, boards, redistribution layers (RDL), and so forth. Thus, substrate and package warpage is an increasing issue, which can create complications during package assembly, surface mount processing, and other manufacturing processes. For example, when substrate layers warp, the surfaces can become uneven and non-planar. This can have a negative impact on reliability and longevity of the packages and related assemblies.